Because the world cup is held in 3 different countries. For example, in Canada we know there is a cold country (generality), but on the other hand, in Dallas, Houston and even Mexico, temperatures tend to be warmer than in Canada, could this be a problem and could it harm some teams ?
I don't think the temperature will be a problem.
Just look at the example of Qatar, where the temperatures were quite high, even playing in there in a different month.
USA alone is a big country whith different temperatures.
Look at temperature averages for June and July:
USA: June: 18° to 28°C - July: 21° to 31°C
And only 20 matches of group stage will be disputed in Canada and Mexico.
So, it's not a problem in my opinion
To attract more people, more money, Canada and Mexico don't need to spend a lot of money in stadiums, infrasctruture etc, with more matches being disputed, this is probably a test to the next cups, and could be the new rule, in 2030 we can see 4 countries of South America or 3 countries of Europe.